1. Field of the Invention
The present invention relates to the preparation of polyester polyols having an acid number less than 1 in a shorter time in comparison to the usual processes; i.e., in higher volume-time-yields.
The present invention further relates to the molten state condensation of polyester-polyol-initial components and the subsequent oxyalkylation of the polycondensates obtained in the presence of special catalysts.
2. Description of the Material Art
Polyester polyols are commonly prepared from polycarboxylic acids and multivalent alcohols through molten state condensation in the presence of acids or metal salts used as catalyst. The polycondensation is preferably carried out under reduced pressure, in order to distill off the water resulting from the condensation from the reaction mixture and to move the equilibrium reaction in the direction of polyether formation. In this manner, for example polyester polyols having acid numbers larger than 5, can be prepared in relatively short reaction times.
However, in many applications, such as in the preparation of polyurethanes, it is necessary that the polyester polyols exhibit uniform molecular weights and low acid numbers. In the preparation of polyester polyols having acid numbers less than 2, and especially less than 1, not only are both long reaction times and high reaction temperatures required, but a further disadvantage is also that the products are darkly colored based on the decomposition reactions, and when reacting with isocyanates they have varying reaction times. In order to overcome these disadvantages, it was proposed to carry out the condensation in the presence of catalysts; for example, iron, cadmium, cobalt, lead, zinc, antimony, magnesium, titanium and tin catalysts, in the form of metals, metal oxides or salts. These compounds, however, not only catalyze the polyester preparation, but also accelerate the hydrolytic decomposition of the polycondensation products in the presence of moisture in the air. Another disadvantage is that the catalyst residues contained in the polyester polyols used in the preparation of polyurethanes, can interfere by forming side reactions, such as allophanate or biuret formation, isocyanate trimerization and isocyanate polymerization, or by addtional catalysis or by hindering the polyaddition.
The process for preparing polyester polyols and their use for the preparation of polyurethane plastics using the polyisocyanate addition polymerization process is well known, and has been described in numerous literature and patent publications. Specifically, reference is made to the Plastics Handbook, vol. 7, Polyurethanes, 1st edition, 1966, published by A. Hochtlen and R. Vieweqs, and the 2nd edition in 1983, published by G. Oertel, Carl Hanser-Verlag, Munich, Vienna.
As disclosed in U.S. Pat. No. 3,907,863, (DE-OS No. 2 316 293) polyester polyols having acid numbers less than 1 can be prepared without catalysts in relatively high volume-time-yields. According to this process, polycarboxylic acids and multivalent alcohols are polycondensed at temperatures from about 150.degree. to 350.degree. C., whereby distillable dialcohols or mixtures comprised of di- and monoalcohols with acid numbers less than 480, and monoalcohols with an acid number less than 60, are incorporated into the esterification mixture. The polycondensation is carried out to the end, while, at the same time, the excess alcohol is removed by distillation. Through the use of mono-alcohols, the acid number and functionality of the polyester polyol can be decreased.
As disclosed in DE-AS No. 1 248 660 (GB 1 139 634), glycol monoesters can be prepared through the esterification of organic mono- and polycarboxylic acids with ethylene oxide in the presence of thio-ethers which are used as catalysts. In U.S. Pat. No. 4,452,997, (EP-PS No. 0 087 517), a process is described for the preparation of polyester- and/or polyesterpolyether polyols in which di- through hexafunctional polyether polyols having hydroxyl numbers of from 30 to 200, are reacted with at least 1 carboxylic acid anhydride to form a carboxylic-semi-ester. This is then oxyalkylated with at least 1 alkylene oxide in the presence of 0.001 to 0.1 moles of from at least 1 thio-dialkylene glycol per carboxyl group of the carboxylic semi-ester. EP-OS 0 124 070 discloses an improvement over the above process by the reaction of di- through hexafunctional polyester polyols having hydroxyl numbers from 15 to 250, with at least 1 carboxylic acid anhydride, in the presence of N-methylimidazole, triethylenediamine, and/or triphenylphosphine. The resulting carboxylic semi-ester is oxyalkylated with at least 1 alkylene oxide in the presence of N-methylimidazole, triethylenediamine, triphenylphosphine, thiodialkylene glycol, or a mixture comprising at least two of the said compounds, which are used as catalysts.
None of the aforementioned European or U.S. patents and/or disclosures disclose the rapid reduction of the acid number of polyester polyols prepared through molten state condensation. Accordingly, the present invention differs from the art listed herein.